In digital data transmission, it is common practice to transmit an acknowledgement (ACK) signal from a receiver to a transmitter to indicate successful reception of a data packet. If a transmitter does not receive an ACK or receives a non-acknowledgement (NACK) signal during a defined time period, the data packet will be retransmitted.
Retransmitting a data packet provides an opportunity to improve the efficiency of the retransmission by using signaling diversity. For example, a data packet may be retransmitted on a different frequency to achieve frequency diversity or a data packet may be retransmitted on a different antenna to achieve spatial diversity. The efficiency of data packet retransmissions may also be improved by signaling different versions of redundancy bits.
Signal mapping diversity is a promising technique to improve the efficiency of data packet retransmissions. To achieve signal mapping diversity, the same bit sequence is mapped onto different signal points in a signal constellation when the modulation is higher than the quadrature phase shift keying (QPSK) in use. It is well known in the art that when modulating bits to symbols of higher order modulation, the reliability of those bits are not equal. The reliability of those bits depends on the location and/or the value of bits.
One scheme of 16 QAM and 64 QAM constellation remapping, proposed for 3GPP, aims to average the bias of bit reliability during data packet retransmission. In addition, a prior art proposal for 16 QAM has been adapted to high speed downlink packet access (HSDPA) standards. This remapping technique is referred to as bit-wise remapping. Bit-wise remapping is effective when a system employs strong channel coding. However, bit-wise remapping performs poorly when a system employs weak channel coding because weak channel coding causes a high code rate and results in a small number of redundancy bits.
Another scheme of signal remapping aims to maximize the minimum combined squared Euclidean distance (CSED) across multiple retransmissions. This criterion effectively minimizes raw symbol error rate. This remapping technique is referred to as symbol-wise remapping. Symbol-wise remapping performs best when a system employs weak channel coding because the system approaches an uncoded system.
Prior art symbol-wise remapping techniques fail to take advantage of the fact that the I and Q labeling bits of a QAM signal are separable. Therefore, a new mechanism that takes advantage of separate I and Q labeling bits in a QAM signal and reduces the complexity of symbol-wise remapping is desired. Further, it has been shown that a bit-wise remapping scheme performs best when the system uses strong channel coding and a symbol-wise remapping scheme performs best when the system uses weak channel coding. Therefore, a method and apparatus for adaptive signal remapping according to channel coding is desired to improve overall performance compared to remapping schemes using a single technique and to reduce the complexity of a receiver.